1. Field of the Invention
This invention relates generally to stored memory packages, and more particularly to an improved rotational field generating apparatus for a bubble memory package.
2. Prior Art
A rotating magnetic field is a preferred method for propagation of magnetic bubbles in a bubble memory package. Customarily, this rotational magnetic field is generated using flat coils or stripline coils, one mounted on top of the other, but physically displaced 90.degree. relative to each other, and are driven by a sinusoidal or triangular current with appropriate phase shift of 90% at a frequency of between one hundred kilohertz and one megahertz to provide the desired inplane rotational field. A typical example of such a rotational field generation package is found in the bubble devices described in the article entitled "Reflection Coil Packaging for Bubble Devices" by Masaki Takasu, Harumi Maegawa, Toshiaki Sukeda and Kazuo Yamagishi of the Fujitsu Laboratories, Ltd., Kawasaki, Japan, which was published in the IEEE Transaction on Magnetics, Volume MAG-11, Number 5, September 1975. In this package, the structure uses an upper pair of flat coils and a lower pair of flat coils, each pair being oriented orthogonal to each other. Then, by selectively varying the phase and magnitude of the current's pulse through the respective coils, an effective rotating magnetic field is produced between the pairs in the area containing the bubble memory chips. A primary difficulty encountered with these and similar prior art bubble memory coil packages are that the coils are expensive, space consuming and quite often hard to assemble. They may exhibit relatively high inductances which complicate operations at high frequencies. In addition, special high-power circuits are needed to meet the start/stop requirements of the systems without overshoot or ringing occurring, and to contain inherent high over-voltages. Generally, rather elaborate shielding of the drive lines, circuitry and coils to protect the sensitive bubble sense circuits is also required.
It is also quite important that the coil characteristics be closely matched and carefully controlled. This becomes especially difficult in that for each pair, one of the coils is further removed from the active zone of the bubble memory chips than the companion coil and therefore to provide the same effect on the rotational magnetic field, its design and/or control compensation must be carefully managed to accomplish the desired magnetic field effects.